1. Field of the Invention
This invention relates to a fluid pressure control apparatus for a vehicle.
2. Description of the Prior Art
Hitherto, when the vehicle is braked, the fluid pressure from the master cylinder is transmitted, as it is, to the wheel cylinders of the front wheels, without reduction, both during forward running and during backward running.
Generally, when the 2L-type drum brake is employed for the front wheel, the braking force applied to the front wheel during forward running is considerably larger than the braking force applied to the front wheel during backward running, for the same brake fluid pressure, on the ground of the construction of the 2L-type drum brake. Accordingly, the forward falling amount of the automobile or vehicle which is braked during forwards running, is larger than the backward falling amount of the automobile or vehicle which is braked during backwards running. The forward falling of the automobile is so called "nose-dive phenomenon". The backward falling of the automobile is called provisionally "tail-dive phenomenon". The falling or swing of the automobile occurs due to the inertia of the center of the gravity of the automobile, when the wheels are braked. The load distributions to the front and rear wheels change due to the rotational force round the contact points between the front or rear wheel and the ground.
On the other hand, when the disc brake type is employed for the front wheel, the braking force applied to the front wheel during forward running is equal to the braking force applied to the front wheel during backward running, for the same brake fluid pressure.
Accordingly, when the 2L-type drum brake is substituted for the disc brake type, "the tail dive amount" becomes considerably large in comparison with the case when the 2L-type drum brake is employed.
A larger suspension mechanism of high quality to which a larger load may be applied, should be used for the rear wheels. Further, the suspension mechanism itself for the front wheels should be so altered as to prevent the above described "nose dive" and "tail dive".
As the result, the whole suspension mechanism for the front and rear wheels should be altered. The change of the brake type for the front wheel requires the alteration of the substantially whole construction of the automobile. However, the alteration merely copes with the phenomenon which occurs during backwards running. That is not so efficient. Further, there occur serveral problems such as the increase of the weight of the automobile and the lowering of the fuel consumption rate.
Also in the case that the brake type is not altered, the braking force obtained in the front wheel becomes considerably large by a relatively strong tread of the brake pedal during backwards running, and so the "tail dive" phenomenon becomes considerably remarkable. A large load is applied to the suspension mechanism. There occur some problems such as the lowering of the endurance and the damage.
In order to reduce the falling amount of the vehicle body by braking operation during backward running, such a fluid pressure control apparatus was proposed that comprises an inlet port connected to a master cylinder, an outlet port connected to a wheel cylinder of a front wheel brake apparatus, a valve apparatus arranged in a path connecting the inlet port with the outlet port, and being able to limit the communication between the inlet and outlet ports, a movable plunger receiving at least the fluid pressure at the side of the outlet port, and being able to open and close the valve apparatus, a spring for urging the plunger in the direction to open the valve apparatus, and an electromagnetic coil to be energized by a signal transmitted on the basis of the driver's operation for driving the automobile backwards, wherein the fluid pressure at the side of the outlet port can be reduced by the energization of the electromagnetic coil.
The above fluid pressure control apparatus further includes a control piston and a second valve apparatus arranged in a path connecting the chambers at both sides of the above plunger. The control piston is moved to close the second valve apparatus by the energization of the electromagnetic coil on the backward running. Thus, the fluid pressure at the side of the outlet port can be reduced by cooperation of the plunger and first-mentioned valve apparatus. Accordingly, the fluid pressure control apparatus is complicated in construction and large-sized.